In semiconductor circuits and devices, bond pads are used to make electrical contact between various components of the circuit or device. For simplicity of discussion, the following discussion focuses on bond pads for imagers; however, the scope of the invention should not be limited as such and may be used for any type of integrated circuit device.
Imagers, including charge coupled devices (CCDs), photodiode arrays, charge injection devices (CIDs), hybrid focal plane arrays, and complementary metal oxide semiconductor (CMOS) imagers, have commonly been used in photo-imaging applications. Current applications of solid-state imagers include cameras, scanners, machine vision systems, vehicle navigation systems, star trackers, and motion detector systems, among others.
A CMOS imager typically includes a focal plane array of pixel cells, each one of the cells including a photosensor, for example, a photogate, photoconductor or a photodiode for accumulating photo-generated charge. Each pixel cell has a charge storage region, which is connected to the gate of an output transistor that is part of a readout circuit. The charge storage region may be constructed as a floating diffusion region. In some imagers, each pixel cell may include at least one electronic device such as a transistor for transferring charge from the photosensor to the storage region and one device, also typically a transistor, for resetting the storage region to a predetermined charge level prior to charge transference.
To allow the photosensors to capture a color image, the photosensors must be able to separately detect color components of the captured image. Accordingly, each pixel must be sensitive only to one color or spectral band. For this, a color filter array (CFA) is typically placed in front of the optical path to the photosensors so that each photosensor detects the light of the color of its associated filter.
Further, since the magnitude of the signal produced by each pixel is proportional to the amount of light impinging on the photosensor, it is also desirable to improve the photosensitivity of the imager by collecting light from a large light collecting area and focusing it onto a small photosensitive area of the photosensor. This can be done using a micro-lens array formed over the pixel array.
FIGS. 1A and 1B illustrate an imager 10 including a color filter array 30 and micro-lens array 35 over a pixel array 25. As can be seen in FIG. 1A, imaging portion 15 may be surrounded by peripheral circuitry 18 and bond pads 20. The bond pads 20 are electrically connected to device circuitry of the imager 10. Peripheral circuitry 18 controls the imaging portion 15 and converts electrical signals received from the imaging portion 15 into a digital image. As can be seen in FIG. 1B, the pixel array 25, peripheral circuitry 18 and bond pads 20 are formed in semiconductor wafer 50. Other conventional parts of the imager 10 are not shown or described herein.
The process for forming the color filter array 30 and/or micro-lens array 35 over the pixel array 25 requires a multi-step fabrication process. A common method of forming these structures includes a tetramethylammonium hydroxide (TMAH)-based developing solution. Typically, TMAH-based developing solutions used for color filter array formation have a TMAH concentration ranging from about 0.6% to about 2.6%. The entire CFA/microlens formation process typically involves about six developing steps where this developing solution comes into contact with the bond pad for approximately 30 seconds to 1 minute per each step. The TMAH contained in this developing solution is very aggressive on the aluminum from which the bond pads 20 are typically formed. As can be seen in FIG. 2, when an aluminum bond pad 20 is exposed to a TMAH-based developing solution 55, it is attacked and slowly etched away, creating pits 60 on the surface of the aluminum, making the surface very rough. Through an entire CFA/microlens formation process, about 100-200 nm of aluminum may be removed from the surface of an unprotected bondpad. This pitting and removal of aluminum can cause problems in later processing and use of the imager. The roughness, caused by the pitting, increases the scattering of light and makes the pads look very dark when viewing the pads through a scope. In addition, the excessive removal of aluminum can cause bonding issues, caused by too much topography in the aluminum layer or by not enough aluminum remaining in the bondpad.
An organic protective layer or an oxide layer can be formed over the bond pads 20 before the CFA/micro-lens formation process to protect the bond pads 20 from the TMAH solution. However, these layers must be removed after the CFA/micro-lens formation process in order to avoid problems (e.g., bonding issues during packaging of the imager) during later processing steps. Furthermore, the oxide layer, for example, must be removed using an expensive hard-coat process.
Accordingly, there is a need and desire for a simpler process by which bond pads may be protected during subsequent processing of a semiconductor device, for example, an imager.